1. Field of the Invention
The present invention relates to a class-D amplifier, and particularly to a class-D amplifier with dual feedback loop.
2. Description of Related Art
Amplifiers can be categorized as class-A amplifiers, class-B amplifiers, AB-class amplifiers and class-D amplifiers. As the development of semiconductor technology, the class-D amplifiers with low power consumption have been applied in widespread applications, such as sound reinforcement system.
In comparison to AB-class amplifiers using linear signals, class-D amplifiers use pulse width modulation (PWM) technique to drive an inductive load device, wherein the PWM technique involves audio signals, PWM switch signals and harmonic signals. The PWM switch signals are applied to alternately turn on and off switching transistors of the class-D amplifier. Because the switches are either fully on or fully off, the power losses in the output devices are significantly reduced to ensure high power efficiency.
With reference to FIG. 5, a conventional class-D amplifier (70) with open loop scheme comprises a gain amplifier (71), a PWM modulator (72), an internal oscillator (73) and an output driver (74).
The gain amplifier (71) has input terminals (Vi+)(Vi−) for receiving an analog audio signal, amplifies the analog audio signal and transmits the amplified audio signal to the PWM modulator (72). Upon the receipt of the amplified audio signal, the modulator (72) refers to an oscillating signal of the internal oscillator (73) to produce PWM signals. The PWM signals controls the output driver (74). The output driver (74) is adapted to connect to an inductive load device (60). The inductive load device (60), for example a speaker, receives the audio signal and restores sound accordingly.
Since the class-D amplifier (70) is configured as an open loop differential structure and the gain amplifier (71) has a differential amplifier (701), random noise floor contributed from the differential amplifier (701) will also be amplified with audio signal. Therefore, when the inductive load device (60) restores the amplified audio signal to sound voice, the sound voice may distort. As a result, the signal to noise and distortion ratio (SNDR) of the class-D amplifier (70) is impacted.
With further reference to FIG. 6, noises (N1) existing in the audio signal can be shaped to higher frequency beyond the signal band, such that the noises (N1) can be eliminated by the low pass characteristic (81) of the low pass filter (80) to retain desired audio data (S1).
With reference to FIG. 7, another conventional class-D amplifier (70a) with close loop scheme has two differential output terminals (Do+)(Do−) and comprises a gain adjusting circuit (711), a first differential amplifier (712), a first-order integrator (75), two comparators (76), a triangle wave oscillator (77), a logic circuit (78) and an output driver (74).
The gain adjusting circuit (711) has input terminals (Vi+)(Vi−) for receiving an analog audio signal.
The differential amplifies (712) is connected to the input terminals (Vi+)(Vi−), and an amplification gain value of the differential amplifies (712) is controlled by the gain adjusting circuit (711).
The first-order integrator (75) comprises a second differential amplifier (751) and two RC circuits. Each of the two RC circuits is connected between one corresponding differential output terminals (Do+)(Do−) and one corresponding input terminals (+)(−) of the second differential amplifier (751). The differential output signal of the class-D amplifier (70a) is feeding back to the second differential amplifier (751) and incorporated with the amplified audio signal output from the first differential amplifier (712).
With further reference to FIG. 8, each comparator (76) has two input terminals, one of the input terminals is connected to a corresponding output terminal (+)(−) of the second differential amplifier (751), and the other input terminal is connected to the triangle wave oscillator (77). Therefore, each comparator will compare the sine wave signal (Vi+)(Vi−) of the first-order integrator (75) with the triangle wave signal (S2) to produce two PWM signals.
The logic circuit (78) is connected to the two comparators (76) to output two driving signals based on the received two PWM signals.
The output driver (74) comprises two half bridge switch circuits (741). The two half bridge switch circuits (741) are controlled by the logic circuit (78) and adapted to drive an inductive load device (60) via the differential output terminals (Do+)(Do−).
Since the class-D amplifier (70a) provides a first-order feedback loop formed by the second differential amplifier (751) and the two RC circuits, the signals to be input to the two comparators (76) are produced by combining an error signal from the second differential amplifier (751), an amplified input audio signal from the first differential amplifier (712), and a real output signal. The amplified input audio signal output from the first differential amplifier (712) contains non-linear components, such as amplifier frequency limitations, amplifier noise, reference voltage noise, gain-bandwidth product limitations and switch device non-linearities. These non-linear components can be eliminated by the first-order feedback loop employing a single integrator.
However, because the class-D amplifier (70a) only uses a single integrator employing a differential amplifier (75) to process the audio signal, the effect of noise rejection the class-D amplifier (70a) through the noise shaping is limited.
To overcome the shortcomings, the present invention provides a class-D amplifier with the dual feedback loop scheme to mitigate or obviate the aforementioned problems.